Comparative anatomy and muscle architecture of selected hind limb muscles in the Quarter Horse and Arab (2024)

Equine hind limb anatomy

The proximal hind limb muscles, many of which attach directly to the skeleton without a tendon (Payne et al. 2005), account for the majority of the horse's locomotor mass (Gunn, 1987; Payne et al. 2005). Anatomical, biomechanical and electromyographic studies indicate that these muscles provide the mechanical power output for activities which require net external work, such as acceleration, running uphill and jumping (Robert et al. 2000; Clayton et al. 2002; Dutto et al. 2004a).

The distal hind limb muscles, small in comparison, attach to the skeleton via long thin spring-like tendons, an adaptation in cursorial animals which reduces the need for energetically expensive muscle length change (Dimery et al. 1986). Further information regarding the locomotor anatomy, origin, insertion and action of the muscles that were studied can be found in Fig. 1 and Table 1.

The role of an individual muscle during locomotion is influenced by several features: muscle architecture; fibre type; moment-generating capacity (muscle force multiplied by moment arm) (); and activity patterns. In this study we focus on breed differences in muscle mass and architecture in the equine hind limb; future studies will address differences in fibre type and moment arms.

Muscle architecture (the arrangement of the muscle fibres within the muscle, relative to the axis of muscle force generation) is often described in terms of the following five parameters: muscle belly and tendon length; muscle fascicle length; muscle fascicle pennation angle; and physiological cross-sectional area (PCSA) (Zajac, 1992). Physiological cross-sectional area (PCSA) is defined as the sum of the cross-sectional area of the muscle fibres within the muscle belly. It can be estimated using the following equation:

where muscle density is taken as 1.06 g cm⁻³ (Mendez, 1960).

PCSA is related to the muscle's maximum isometric force potential (F_max) (Powell et al. 1984), because F_max is a product of PCSA and the maximum isometric stress of vertebrate skeletal muscle. However, owing to pennation of the fascicles, only a proportion of the fascicle force acts in the direction of the muscle belly and tendon. Thus F_maxis estimated by:

where the maximum isometric stress of vertebrate skeletal muscle is assumed to be 0.3 MPa (Wells, 1965; Woledge et al. 1985; Zajac, 1989; Medler, 2002), and θ is the pennation angle of the muscle fascicles.

Fascicle length (which is indicative of the number of sarcomeres arranged in series within the fascicle) relates to the range of motion over which a muscle can exert its force and its velocity of contraction (Gordon et al. 1966; Wickiewicz et al. 1984).

Muscle architecture has been shown to vary between muscles within the same individual (Brown et al. 2003; Payne et al. 2005), and between the same muscle in different individuals (Abe et al. 2000; Blazevich et al. 2006). Exercise (Blazevich et al. 2003) and genetic make-up, in terms of gender (Abe et al. 1998) are known to influence muscle architecture, but the extent to which differences in training and genetic profile relate to athletic performance remains unclear.

When compared to elite distance runners, human elite sprint runners have been observed to have longer muscle fascicle lengths, arranged at lesser pennation angles in their leg muscles (Abe et al. 2000), which has been linked to running speed (Kumagai et al. 2000).

Research into the effect of differences in muscle architecture and performance in the horse is sparse; instead researchers have tended to focus on the effects of different muscle fibre types (; Rivero et al. 2001) and metabolic influences such as the maximum capacity for oxygen consumption by the body during maximum exertion (VO_2max) (Langsetmo et al. 1997; Prince et al. 2002). This paper attempts to address this gap in the literature by comparing the hind limb muscle architecture in two breeds of horse – the Quarter Horse, which has been selectively bred for sprinting, and the Arab, which has been selectively bred for endurance – to see whether differences in breed athleticism are reflected in differences in muscle architecture. Whilst all animals were being regularly exercised, none was being trained for either sprint or endurance activities; as such it is expected that any observed differences in muscle architecture will be as a result of breed differences rather than training effect.

It was hypothesized that, when comparing horses of similar height and body mass, the Quarter Horse hind limb muscles would, firstly, have a greater mass and, secondly, have longer fascicle lengths with lesser pennation angles when compared to the same muscles in the Arab.

Horses

The study was conducted at the University of Queensland and approved by the University's Animal Ethics Committee. Twelve adult horses (minimum age 3.5 years) were euthanased for reasons unrelated to musculoskeletal disease (Table 2): six Quarter Horses, consisting of four geldings and two mares (13 ± 8 years); and six Arab horses, consisting of five geldings and one mare (13 ± 7 years). All horses had previously been in light ridden work.

The height (m) of each animal was determined by the use of a tape measure, measuring from the ground up to the highest point of the withers whilst the horse was standing square, sedated in stocks. All of the subjects were unshod. The mass (kg) of each subject was determined when feasible by standing the animal on an electronic weigh scale (‘Tru Test Easy Weigh II’ Sunbeam, Sydney, Australia). Limb length was determined by measuring and combining femur, tibia and the third metatarsal bone (MT3) length once the muscles had been removed. Bone lengths were measured using standard anatomical landmarks. Femur length was measured from the most proximal part of the greater trochanter to the most distal point on the lateral femoral condyle; tibia length was measured from the inter-condylar eminence to the most distal point on the medial malleolus; and MT3 length was measured from the most proximal lateral part of the head to the most distal part of the lateral condyle. It was not possible to measure the bone lengths of the individual phalanges as these distal bones were used in a separate study and hence were not available.

Muscle architecture

Immediately following euthanasia the hind limb was skinned and the overlying fascia was removed to expose the superficial muscles. The superficial gluteal and tensor fascia lata muscles were dissected away from their fascial and bony attachments to expose gluteus medius. The following muscles, together with their tendons, were identified and removed for further analysis: gluteus medius; biceps femoris; semitendinosus; vastus lateralis; gastrocnemius; extensor digitorum longus; and tibialis cranialis. The three heads of biceps femoris and the two heads of gastrocnemius were separated from each other by carefully dividing adjoining fascia.

Muscle belly length (mm) was determined by measuring the distance from the origin of the most proximal muscle fibres to the insertion of the most distal fibres with a flexible tape measure. Muscle mass (g) to the nearest 0.1 g was recorded using electronic scales (EKS, Hereford, UK) and reported to the nearest gram. Muscles over 2000 g were weighed in several pieces.

A minimum of three (central, medial and lateral) full thickness cuts were made in the muscle belly, in a plane following the direction of the muscle fascicles. Five measurements of fascicle length were obtained from each section, at varying depths, by laying a flexible measuring tape along the length of each fascicle. Five resting fascicle pennation angles (; Payne et al. 2005) were determined by measuring the angles between the muscle fascicle and internal tendon and/or aponeurosis using a clear plastic protractor (see plate 1). Multiple measurements within the same muscle and, where appropriate, within muscle compartments, were averaged. If resting pennation angles were less than 5°, they were given a value of zero and were not considered in calculations of F_max.

Statistical analysis

The Mann–Whitney U-test, at a significance level of P < 0.05, was used to compare the height, body mass, hind limb bone length, individual muscle mass and muscle belly lengths between the two subject groups. A two-way hierarchical anova at a significance level of P < 0.05 was used to determine whether statistical differences in fascicle length and pennation angles were present between the two subject groups. Data were tabulated using MicrosoftOfficeExcel 2003 (Microsoft Corporation, Redmond, WA, USA). Statistical analysis was performed using SPSS, version 14 (SPSS Inc., Chicago, IL, USA).

Height and body mass

It was not possible to detect a statistical difference in the height (m) (P = 0.09, n = 12) or body mass (kg) (P = 0.33, n = 8) between the two groups (Table 2).

Hind limb bone length

No statistical difference was detected in femoral (P = 0.07, n = 12) or third metatarsal bone lengths (P = 0.62, n = 12), but tibial bone length was slightly greater in the Quarter Horse group (P = 0.015, n = 12) (Table 2).

Muscle mass

The Quarter Horses exhibited larger muscle masses (Table 3), and hence muscle volumes, in both the proximal and distal muscles (P ≤ 0.01, n = 12). Gluteus medius, which had the largest mass in both breeds, represented 2.0% of body mass in the Quarter Horse and 1.7% in the Arab. Gastrocnemius had the largest mass of the distal leg muscles, the lateral head having a greater mass than the medial. Tibialis cranialis, with the smallest muscle mass, represented 0.08% and 0.05% body mass in the Quarter Horse and Arab respectively.

Muscle belly length

The muscle belly lengths of gluteus medius, semitendinosus (vertebral and pelvic heads), biceps femoris (vertebral head), and extensor digitorum longus muscles were statistically longer (P < 0.05, n = 12) in the Quarter Horse group (Table 3).

Fascicle length

No statistical difference (P = 0.20, n = 12) in fascicle length was observed when comparing the same muscle in either breed of horse (Table 3). The proximal muscles in both groups, with the exception of the proximal part of tibialis cranialis, had longer fascicle lengths than distal leg muscles.

Regional differences were observed in fascicle length within individual muscle bellies. Specifically, the medial head of gastrocnemius had longer fascicle lengths than the lateral head, and the proximal part of tibialis cranialis had longer fascicle lengths when compared to the distal part.

Pennation angle

There was no significant difference (P = 0.61, n = 12) in the fascicle pennation angles when comparing the same muscle in either breed (Table 3).

Biceps femoris could be divided into three distinct compartments, named according to their anatomical positioning: the vertebral, intermediate and caudal heads (Payne et al. 2005).

In both breeds, the muscle fascicles were parallel in the middle and caudal heads and had a similar pennation angle in the vertebral heads. Similarly, semitendinosus could be divided into two distinct sections: the pelvic head, which consisted of parallel muscle fascicles (with a pennation angle of less than 5°); and a pennate vertebral head. Tibialis cranialis was composed of parallel fascicles proximally and pennate fascicles in the distal section. There was no statistical difference in the pennation angles of gastrocnemius medialis or lateralis in either breed or between the two breeds.

Estimated parameters

Proximal-to-distal muscle architecture

The proximal muscles in both breeds of horse had larger volumes (Table 4) and fascicle lengths when compared to the more distal pennate muscles (with the exception of the proximal part of tibialis cranialis muscle). These findings are in agreement with previous studies of equine hind limb muscles (Payne et al. 2005).

Muscle mass

In this study, the Quarter Horse hind limb muscles contributed a higher percentage to total body mass when compared to those of the Arab, but the relatively small sample (n = 4) makes it difficult to draw any firm conclusions. However, a similar study by Gunn (1987) found the hind limb muscle mass to body mass ratio to be greater in thoroughbred horses trained for sprinting when compared to hurdling, suggesting this attribute may well be related to the ability to accelerate rapidly.

Muscle belly length

The Quarter Horse hip extensor (gluteus medius, semitendinosus and the vertebral head of biceps femoris) and extensor digitorum longus muscles had significantly longer muscle bellies when compared to the same muscles in the Arab (Table 3). This may be owing to the fact that the Quarter Horse group had slightly longer limb lengths (afforded by a longer tibia), although the remaining muscles in both groups were of a similar belly length. It may alternatively/additionally be due to conformational differences between the breeds: for example, differences in muscle–tendon length (which were not determined), or differences in muscle moment arms (which will be the subject of a future study). It is suggested that the longer muscle belly lengths of the Quarter Horse hind limb retractors may be a reflection of the breed's ability to accelerate rapidly, as the longer muscle belly lengths enable the muscles to generate force throughout a greater range of motion than those of the Arab, facilitating increased stride length (i.e. speed = stride length × stride frequency).

Fascicle length/pennation angle

As it was not possible to detect a significant difference between the two breeds of horse in muscle fascicle length or pennation angle in either the proximal or distal leg muscles, we rejected the hypothesis that the Quarter Horse hind limb muscles would be composed of longer fascicle lengths with lesser pennation angles when compared to the same muscles in the Arab. This hypothesis was proposed based on the fact that longer muscle fascicle lengths (associated with a greater velocity of contraction), arranged at lesser pennation angles, have been observed in the calf muscles of human elite sprint runners when compared to elite distance runners (Abe et al. 2000) and that fascicle length appears to be linked to running speed (Kumagai et al. 2000). Our study, however, suggests that the observed differences in muscle fascicle length and pennation angle in the leg muscles of human elite sprint and endurance runners may have occurred as an adaptive response to a specific training programme, in the same way that muscle mass increases in response to resistance training (Kearns et al. 2000) and pennation angle and PCSA increases in human weight lifters (Aagaard et al. 2001).

Pennation of the muscle fascicles influences the amount of force that is transmitted through the attaching tendon (Zajac, 1992). Greater pennation angles result in a significant force reduction, whilst smaller pennation angles have minimal effect [because in muscles with pennation angles of 20° and below, cos(θ) is close to 1.0]. Although increased pennation decreases force due to the direction of the force application, it also allows muscles to have a larger PCSA for a given volume and therefore results in an increased capacity of the muscle to exert force. In this study, the intermediate and caudal heads of biceps femoris, the pelvic head of semitendinosus, and the proximal part of tibialis cranialis had muscle fascicle pennation angles of less than 5°. The remainder of the muscles were pennate and many exhibited pennation angles in excess of 40°. The pennation angles of all the muscles, in both breeds, are greater than those recorded in the Thoroughbred by (Payne et al. 2005). These differences may be breed-dependent, or because of differences in sampling technique: for example, a different location/compartment within the muscle may have been sampled in the two studies. This is especially relevant considering that pennation angle can vary dramatically throughout the same muscle, a feature particularly evident in the tibialis cranialis muscle. This variation in pennation angle coupled with differences in fascicle length suggests that functional specialization exists within the same muscle, a finding that has been mirrored by other researchers investigating human muscle architecture (Aagaard et al. 2001; Blazevich et al. 2006). More detailed studies of individual muscles would provide important information on muscle morphological compartmentalization and hence functional compartmentalization.

It is possible that differences in study design make it inappropriate to compare the findings of our research to published human studies which have determined differences in muscle architecture in vivo using B-mode ultrasound (Kumagai et al. 2000; Kearns et al. 2000; Blazevich et al. 2006). Undoubtedly, ultrasound allows the dynamics of muscle architecture to be examined in vivo, enabling the recordings of fascicle lengths and pennation angles at specific muscle lengths (joint angles) and levels of muscle tension (Narici et al. 1996; ); however, it is constrained to providing information on superficial musculature only and hence would not have been a viable method of obtaining architectural data in this study. We determined fascicle length and pennation angle after the muscles had been removed from the limb. It is possible that subsequent changes in fascicle length occurred, especially following rigor mortis and potential dehydration of any of the muscles, which can result in tissue shrinkage (Cutts et al. 1991). However, as all muscles in this study were examined and compared within 48 h of euthanasia, under the same conditions, breed differences in resting fascicle length and pennation angle should not have been affected. It is appreciated that under ideal circ*mstances muscle fascicle length should have been normalized to sarcomere ‘resting’ length, but this was not possible in this study.

Calculated parameters

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